This invention pertains to a method for preparing a water-containing vinyl acetate polymer dispersion, which comcomprises adding vinyl acetate monomer and an initiator to a water solution from an emulsifier and hydroxypropyl and/or hydroxyethyl starch.
Said substituted starch acts as protective colloid.
The preparing of dispersions from homo- and co-polymers of vinyl acetate are already long known.
Said dispersions are notably used as adhesives in the wood and paper industries, in coatings for paper, in the paint industry and as reinforcing for book backs.
To prepare such dispersions there is started with a water phase wherein an emulsifier, a salt as buffer and generally also a protective colloid are dissolved or dispersed and with a monomer or a monomer mixture.
The water phase is heated to the polymerizing temperature and some initiator is added.
After some time, long enough to let said initiator form radicals, monomer is added.
After completing the addition, post-heating is performed during one to two hours. The reaction mixture is cooled and possibly filtered. The dispersions or latexes have generally a content in dry material from 48 to 65% based on the total weight but dispersions with a lower content in dry material can also be prepared in this way.
The polymerizing reaction of vinyl acetate follows a free-radical mechanism. The radicals are provided by the heat-decomposition from an initiator, for example potassium, persulphate, ammonium persulphate or organic peroxides.
A protective colloid is possibly used for preparing such dispersions. Protective colloids increase the viscosity and thus act as thickening agents. They further increase the stability of said emulsion during the reaction and thus oppose coagulating or flocculating. The protective colloids increase after the reaction, the mechanical stability of the dispersion or latex and thus oppose coagulating or flocculating during conveying, storing or further treatment. The protective colloids also increase the freezing-thawing stability and the general stability of the dispersion.
Besides substituted starch, the following are further used up to now as protective colloids: partly or completely hydrolyzed polyvinyl alcohols, natural and synthetic gums and cellulose ethers, such as hydroxyethyl cellulose and hydroxyethyl-methyl cellulose.
All of the above products are water-soluble and give viscous solutions which act as colloidal media.
The polyvinyl alcohols have the drawback of not being resistant in the presence of borax. Natural and synthetic gums are expensive and are of varying quality depending on the origin thereof. The cellulose ethers precipitate by heating at a temperature which is dependent on the product, from the dispersion they are absorbed in.
Starch derivatives have the advantage not to precipitate by heating from the solutions thereof and to be resistant in the presence of borax.
The use of starch derivatives and notably of hydroxypropyl and hydroxyethyl starch as protective colloids for the preparation of dispersions from homo- and copolymers of vinyl acetate is known from U.S. Pat. No. 3,769,248. It has been proposed therein to use starch derivatives with a degree of substitution from 0.05 to 0.75 hydroxyethyl or hydroxypropyl groups per anhydroglucose unit.
From the embodiment examples in the above Patent there further appears that the dispersion preparation requires a large amount from emulsifier. It did now surprisingly appear that by using substituted starch with another degree of substitution as defined hereinafter, the optimum protecting and thickening action of the starch derivates occurs with a smaller amount from emulsifier.